Tenderness is a primary attribute of meat, such as poultry. Deboning muscles, which have not yet developed full rigor mortis, can cause them to contract and shorten, for example, due to energy left in the tissue. This can result in tough meat. Poultry, such as chicken, typically takes about six hours to develop full rigor mortis. However, in-line processing and portioning of poultry is a continuous process that typically permits deboning at approximately three hours post mortem—often well before rigor mortis has fully set in. Therefore, in order to preserve the full tenderness of the meat, avoid contamination of the meat, minimize process times, and obtain other benefits, it is desired to expedite the onset of full rigor mortis in the meat.
Rigor mortis is the process of cell death. When an animal dies, its individual cells remain alive, using the energy stored in them. At death, with the loss of blood that supplies oxygen to the cells, the cells gradually shift from oxygen dependant (aerobic) metabolism to oxygen independent (anaerobic) metabolism. The cells continue to use energy, but make it more slowly because oxygen independent metabolism is less efficient than oxygen dependant metabolism.
This imbalance of energy production causes the cells primary energy compound, adenosine triphosphate (ATP), to decrease. The result of this metabolism causes the metabolite lactic acid to accumulate. In living animals, lactic acid would be removed by the blood supply. In a dead animal lactic acid accumulates in the muscle. This accumulation of lactic acid causes the pH of the muscle to drop from neutral (7) to about 5.7. At a pH of 5.9 the muscle is in the beginning stages of rigor. This drop in the pH results in further ATP reduction.
ATP has several functions in the muscle. One function is to regulate the interactions of the protein fibers involved in contracting muscle fiber, and in relaxing the muscle fiber back to the original length. When ATP is depleted, the muscle is in rigor mortis. When ATP is reduced, the muscle can no longer relax, or become tender.
Keeping the bone or skeletal structure in tact until rigor mortis is reached prevents the muscle fiber from contracting to a level that would cause toughness. However, cutting and deboning the muscle before rigor mortis is developed will cause a nervous signal in the muscle and cause it to contract more. A more contracted muscle is tougher, for example, because it is thicker.